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DBMS: WAL protocol Leave a reply

WAL protocol Write-ahead logging (WAL) is aUsually both redo and undo information is stored in the log.

Before a block of data in main memory can be output to the database (in nonvolatile storage), all log records pertaining to data in that block must have been output to stable storage. This rule is called the WAL rule. Strictly speaking, the WAL rule requires only that the undo information in the log have been output to stable storage, and permits the redo information to be written later. The difference is relevant in systems where undo information and redo information are stored in separate log records

Write-ahead logging is employed to flush log records to the persistent log file before data pages are written or at commit time

Imagine a program that is in the middle of performing some operation when the machine it is running on loses power. Upon restart, that program might well need to know whether the operation it was performing succeeded, half-succeeded, or failed. If a write-ahead log were used, the program could check this log and compare what it was supposed to be doing when it unexpectedly lost power to what was actually done. On the basis of this comparison, the program could decide to undo what it had started, complete what it had started, or keep things as they are.

This entry was posted in Data Base on November 30, 2012.

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